A question

I am fairly new to playing a lock again, it's been since BC or Wrath, so please no hate if this is a stupid question. I just want to learn how to play this class correctly again. I'm mostly just venturing into LFR's atm. I know we can refresh our dots when they have a certain amount of seconds left to them without losing any time, is it better to say reapply corruption when it has 6 sec. remaining or is it better to let that dot tick to it's last second? I was under the impression that the last few ticks from corruption and agony deal more damage than it's previous ones? But this is old information... help would be appreciated, thanks

You can reapply dots under 50% of their base duration, thanks to a lvl 90 talent Pandemic (well actually this talent allows you to reapply dots, so that their maximum duration is baseduration + 50% baseduration).
It only pays of to reapply dots in the last second, if the dot, which is already applied, was cast with some trinketprocs. But i recommend you to take 5 minutes and read the guide at the top, because it is all well explained in there and these questions already answered.

Only Agony builds damage with later ticks, meaning you should never let that fall off if you can prevent it.

Corruption and UA do constant damage, meaning it's fine to drop them for 1/2s if your previous dot is way stronger that what you can apply now. It's also not a disaster to let them drop in other cases, but it's not optimal then.